Thursday, November 17, 2005

Winter Index Library

This page will contain information on the important indices of winter weather: the NAO (North Atlantic Oscillation) and the ENSO (El-Nino Southern Oscillation). You will be able to check these sites and the associated graphics on a regular basis to see what these winter indicators are at.

Note that negative NAO values trend towards colder weather (and positive trends towards warmer) for the East Coast. Trends going from one direction to another sometimes are accompanied by a significant storm. The El-Nino typically leads to a significant period of moderating weather over the Central and Northern US with significant storms impacting California and the southern US. El-Ninos typically last months, versus a two to three week period of oscillation with the NAO.

One of the most prominent teleconnection patterns in all seasons is the North Atlantic Oscillation (NAO) (Barnston and Livezey 1987). The NOA combines parts of the East-Atlantic and West Atlantic patterns originally identified by Wallace and Gutzler (1981) for the winter season. The NAO consists of a north-south dipole of anomalies, with one center located over Greenland and the other center of opposite sign spanning the central latitudes of the North Atlantic between 35°N and 40°N. The positive phase of the NAO reflects below-normal heights and pressure across the high latitudes of the North Atlantic and above-normal heights and pressure over the central North Atlantic, the eastern United States and western Europe. The negative phase reflects an opposite pattern of height and pressure anomalies over these regions. Both phases of the NAO are associated with basin-wide changes in the intensity and location of the North Atlantic jet stream and storm track, and in large-scale modulations of the normal patterns of zonal and meridional heat and moisture transport (Hurrell 1995), which in turn results in changes in temperature and precipitation patterns often extending from eastern North America to western and central Europe (Walker and Bliss 1932, van Loon and Rogers 1978, Rogers and van Loon 1979).

Strong positive phases of the NAO tend to be associated with above-averagel temperatures in the eastern United States and across northern Europe and below-average temperatures in Greenland and oftentimes across southern Europe and the Middle East. They are also associated with above-average precipitation over northern Europe and Scandinavia in winter, and below-average precipitation over southern and central Europe. Opposite patterns of temperature and precipitation anomalies are typically observed during strong negative phases of the NAO. During particularly prolonged periods dominated by one particular phase of the NAO, anomalous height and temperature patterns are also often seen extending well into central Russia and north-central Siberia.

A meteorological view of the ENSO phenomena offers some answers. A meteorological point of view does not explain ENSO itself; for that one needs to account for the origin of the oceanic conditions, and the coupled interaction of the ocean and the atmosphere is central to that problem.

So how does the atmosphere "know" about El Niño? It is useful to imagine a chain of atmospheric processes, with each link in this chain carrying information from the local vicinity of the El Niño sea surface temperature (SST) anomalies throughout the global climate system. The first link is the tropical response of rain-producing cumulonimbus; critical because deep convection is the principal agent for exchanging heat from Earth's surface and thereby communicating El Niño's presence to the free atmosphere. Wet tropical climates tend to coincide with the warm pool SST area in the western Pacific, and the continental monsoons. During El Niño, rainfall increases over a distance of several thousand kilometers along the equator from the central to the eastern Pacific in response to the warming of the underlying SSTs, as shown in the accompanying figure. Reduced rainfall occurs on the periphery of this wet zone, and even the continental monsoons are not spared ENSO's influence. The opposite effect tends to be experienced during La Nina, although the west-east scale of rainfall anomalies over the equatorial Pacific is somewhat reduced compared to warm events.